PROJECT SUMMARY Many cancer survivors suffer from long-term health challenges, including the threat of cancer recurrence and secondary malignancies, decreased physical functioning, and a markedly increased risk of comorbidities such as obesity, type 2 diabetes, hypertension, bone loss and osteoporosis, and cardiovascular disease. Research suggests that physical activity protects against these problems in cancer survivors, with some of the strongest evidence to date observed in breast cancer survivors. The long-term goal of our work is to advance the science of physical activity promotion in cancer survivors. The overall objective of this project is to develop a novel intervention for physical activity promotion in insufficiently active breast cancer survivors who have completed primary treatment. This behavioral intervention will employ several noteworthy innovations to increase physical activity to recommended levels, including (1) being delivered electronically and (2) incorporating principles and behavior-change techniques from the field of behavioral therapy. The intervention will feature acceptance- and mindfulness-based physical activity-promoting content grounded in the principles of Acceptance and Commitment Therapy (ACT). ACT is a behavioral therapy that supplements skills training with cognitive strategies to increase psychological flexibility: one?s ability to accept and persist despite uncomfortable sensations, thoughts, and feelings that may accompany adherence to valued behaviors. ACT has risen to prominence in the field of mental and behavioral health and has been established as a useful therapeutic approach for effecting behavioral change linked to a wide range of health outcomes. Emerging evidence suggests that electronically delivered interventions derived from ACT tenets hold promise for addressing public health priorities such as smoking cessation and physical activity promotion. The first Aim of this study will be to develop this intervention via an iterative design process that will explicitly ensure the final product?s scientific rigor, relevance to the priority population, and design usability. We will develop content with (1) collaborative feedback from an expert panel and (2) qualitative and quantitative feedback from insufficiently active breast cancer survivors who experience intervention content. In Aim 2, we will pilot test the developed intervention in this population to determine its acceptability (i.e., its perceived utility, perceived ease of use, ability to generate intrinsic motivation). We will also explore changes in participants? physical activity and related cognitions to inform our next steps, which may include testing intervention efficacy with a more robust study design. A detailed training plan is in place to develop the applicant?s quantitative and qualitative data analysis skills, proficiency in scientific communication, and fluency in navigating career-related opportunities. The fellowship will benefit from the guidance of an esteemed mentor with whom the applicant has a strong track record of collaboration, as well as the outstanding environment and resources of a top school of public health and a world-class cancer-specific treatment and research center in the Texas medical center.